1. Field of the Invention
The invention relates to an airbag module with a multi-stage gas generator.
2. Description of the Relevant Art
When using a multi-stage gas generator in an airbag module the stages are ignited individually or successively in dependence on certain factors, such as e.g. severity of crash, seat position and weight of the occupant. After the ignition of a first stage a gas bag, as used for single-stage gas generators in various different designs, would become fully unfolded whereby the gas bag is loaded with a low internal pressure owing to the small amount of gas produced in this stage by the gas generator. In the event of the occupant striking against this gas bag the danger arises that he is not adequately restrained but becomes injured against parts of the vehicle lying behind the unfolded gas bag.
In order to avoid this disadvantage it is known to unfold the gas bag to different sizes in dependence on the ignited state of the gas generator so that the internal pressure of the gas bag is already large enough after ignition of the first ignition stage. Thus, the gas bag after ignition of the first stage is unfolded only to a slight extent but nevertheless up to such a size that it can protect an out-of-position occupant in the event of impact.
Various different solutions are known in order to achieve the different degree of unfolding. Thus a gas bag is known from DE 298 04 239 U1 for a vehicle passenger restraint system where folded areas of the gas bag wall are stitched by rip seams. These rip seams rip open in the event of different internal pressures inside the gas bag so that the volume of the gas bag increases step by step as the internal pressure rises.
Furthermore from DE 298 05 217 U1 a vehicle passenger restraint system is known where at least one arrester strap is provided inside the gas bag to influence the configuration of the gas bag in the unfolded state. For this a winding device is provided on which the arrester strap is housed at least in part in the inoperative state. The winding device has a brake mechanism which in a path- or time-controlled manner after a certain deployment of the gas bag can produce a controlled braking of the arrester strap and thus of the wall of the gas bag up until the stationary state.
The drawback with these known measures is that the gas bag is more complicated and thus more expensive than conventional gas bags.
From U.S. Pat. No. 5, 460, 402 an airbag module is known having a single-stage gas generator in which a gas bag cover is fixed at the side on the edge of the housing by several adhesive spots. As the gas bag unfolds first one side adhesive area of the gas bag cover is separated from the associated adhesive surface so that the gas bag cover opens up on one side. As the gas bag unfolds further the other adhesive surfaces of the gas bag cover also become separated. Through this design the gas bag cover experiences a predetermined opening characteristic which is particularly suitable for determining the specific direction of the gas bag deployment and for reducing the speed with which the gas bag unfolds. A multi-stage gas generator is not described.
The object of the present invention is to obtain at reduced cost a variable unfolding of the gas bag in dependence on the relevant ignited stage of the gas generator.
This is achieved, according to the invention, through an airbag module wherein the gas bag in the inoperative state lies in a folded layer next to the gas bag or a diffuser of the gas bag and only one gas bag layer extends over same so that after ignition of the gas generator the gas bag is drawn only successively out from the housing of the airbag module through the gases discharged from the generator. On the other hand the gas bag cover and/or a part engaging on the folded gas bag is designed so that after the gas bag cover opens from a predetermined time point onwards increasingly greater forces are necessary for the deployment of the gas bag and the outer parts of the gas bag only become unfolded in the event of higher forces.
It was found that through the solution according to the invention it is possible to achieve the same advantageous effect of the variable unfolding as when arrester straps and rip seams are used. Thus no changes have to be made to the gas bag compared to a gas bag for a single-stage gas generator. Folding the gas bag so that after ignition of the gas generator the gas bag is drawn successively out from the housing through the gases discharged, instead of being pressed completely out from the housing, is known per se and requires no additional expense. The required change to the gas bag cover or a part additionally engaging on the gas bag incurs only a slight additional expense.
The gas bag cover is designed according to the invention so that it can be ripped open with increasingly more difficulty from a predetermined point. This predetermined point can be directly adjoining the rip seam of the gas bag cover but can also be provided at a certain distance therefrom. When the gas bag unfolds after ignition of the first stage of the gas generator the gas bag cover is not yet torn fully open owing to the lower pressure in the gas bag. It could be fixed that the gas bag thereby only unfolds in part and that the internal pressure of the unfolded section of the gas bag is however sufficient in order to protect an occupant adequately in the event of impact against the gas bag.
The increasing effort required to tear open the gas bag cover can be achieved in different ways. Thus in a first embodiment it is proposed that the thickness of the cover increases at least in sections starting from a predetermined point with increasing distance from the rip seam. With a particular design of this embodiment the thickness of the gas bag cover increases continuously starting from the rip seam.
In a further embodiment radially aligned incisions are provided as rip lines whose depth decreases away from the centre.
In a third embodiment starting from a central area there are incisions aligned at least approximately parallel and whose depth decreases more and more away from the central area.
In a fourth embodiment at least one reinforcement rib is provided on the gas bag cover from a predetermined point and having at least in sections increasing height as the distance from a rip seam increases. In a special configuration the reinforcement rib has a continuously increasing height.
In a further embodiment the gas bag cover has at least one reinforcement which engages in a fold of the gas bag. This reinforcement represents an additional obstacle in order to prevent a part of the gas bag from unfolding in the event of ignition of for example the first stage of the gas bag. Preferably at least a circumferential reinforcement is provided which engages in at least one circumferential fold.
Another development of the gas bag cover is characterised in that this has a section which even after opening of the gas bag cover through the unfolding gas bag covers a partial area of the housing for storing the folded gas bag.
In a further embodiment the folded gas bag is enclosed at least in part by a foil which becomes more difficult to tear the further away from the rip seam. With a foil of this kind the same effect is achieved as with the cover designed according to the invention or its action is assisted by same.
A gas bag which is folded so that after ignition of the gas generator it is drawn successively out from the housing of the airbag module through the gases discharged from the generator lies in one embodiment in a folded layer next to the gas generator or diffuser and only one gas bag layer extends over same.